(1) Field of the Invention
The present invention relates to electric drive transmissions and more particularly to bearing arrangements therefor. It will be described hereinafter in terms of its application within an overall drive configuration for a battle tank, bulldozer or other skid steered vehicle of the kind described in WO-02/083483. This is not, however, to be construed as limiting the scope of the invention as defined below, which may be found more generally useful where an electric motor is required to deliver torque via a gear change mechanism to a shaft which may be coupled to a load or loads at either axial end of the motor.
(2) Description of the Art
U.S. Pat. No. 4,998,591 discloses in FIG. 3 an electric drive transmission for a tracked vehicle where an electric propulsion motor is arranged coaxially around a main transverse shaft which is coupled at its opposite ends via differential gears to the driven tracks. The motor has a hollow shaft arranged coaxially around the main shaft. A reduction gear or shift gear is also arranged coaxially around the main shaft and connects the motor shaft to the main shaft via a hollow connecting shaft. The bearing arrangement for the various shafts is not disclosed. It would be conventional, however, for the motor shaft to be supported by bearings in the motor casing and for the main shaft to be supported by bearings in some other fixed part of the vehicle structure.
For high power density, electric propulsion motors are often designed to run at high rotational speeds (e.g. in the region of 12,000 RPM). Operating the arrangement of U.S. Pat. No. 4,998,591 with such a high speed motor would be problematical, however. By running the main shaft through the motor shaft the diameter of the latter must necessarily be greater than that of the main shaft, and it must necessarily be of equivalent torque capability. If borne within the motor casing it also follows that the motor shaft bearings must be of substantially larger diameter than the main shaft bearings. Achieving high rotational speeds with large diameter bearings is difficult, however, due to the high rolling speed of the bearing elements, the heat so generated, and other dynamic effects. In practical terms this is therefore likely to impose an undesirable constraint on the operational speed and hence power density of the propulsion motor in the indicated prior art arrangement.
It is one aim of the present invention to overcome the above-noted drawback of the prior art and accordingly in one aspect the invention resides in an electric drive transmission comprising:                a casing;        an electric motor comprising a stator fixed in the casing and an associated rotor borne within the casing;        a separate through shaft passing coaxially through the rotor for delivering torque from the motor to one or more loads; and        a gear change mechanism located within the casing and adapted to transmit torque from the rotor to the through shaft at selected gear ratios;        the through shaft being rotationally supported by bearings acting between said shaft and the casing; and        the rotor being rotationally supported by bearings acting between the rotor and the through shaft.        
It is to be understood that the aforesaid bearings may act directly between the stated components or indirectly through other components of the transmission.
By supporting the motor rotor on the through shaft rather than in separate bearings to the casing the diameter of the requisite rotor bearings can be reduced. In addition, depending on the gear ratio selected through the gear change mechanism, the speed differential (if any) between the rotor and through shaft will be substantially less than that between the rotor and casing, meaning that the speed rating of the rotor bearings can be reduced accordingly. In these ways the problems of high speed large diameter bearings may be avoided. Furthermore by reducing the number of bearing points in the casing the complexity, mass and cost of the casing and the overall size of the transmission can all be reduced.
In another aspect the invention resides in a drive configuration for a skid steered vehicle comprising:                a respective drive member at each side of the vehicle;        a pair of electric drive transmissions according to the first aspect of the invention;        a controlled differential device located between said transmissions; and        a steer motor coupled to the controlled differential device;        the through shaft of each said transmission being coupled at one end to turn a respective said drive member; and        said through shafts being coupled at their opposite ends to respective sides of the controlled differential device.        
It is to be understood that the coupling of the through shafts at either end may be direct to the stated components or indirect through other components of the configuration.
The two said transmissions may share a common casing which also houses components of the controlled differential device.